The present invention concerns a method in a machine for the manufacturing of paper or board, for heating an outer face of such a cylinder or roll which is in direct contact with the web to be pressed against said roll face. The web is treated, such as being dewatered or calendered, by means of the method.
The present invention also concerns a device for the press treatment of a paper web intended for carrying out the method in accordance herewith, this device comprising a cylinder or roll having an outer face which can be heated, and in connection with which one or several roll nips and/or so-called extended nips are formed.
As is known in the prior art, water can be removed from a paper web by means of pressing, so that the dry solids content of the web is ka.congruent.40 . . . 45%. The rest of the water has had to be removed by evaporation, which consumes essentially more energy per unit of mass than is consumed by dewatering by pressing.
In a manner known in the prior art, attempts have been made to intensify the dewatering of the paper web by raising the temperature of the web passing to the pressing, and of the water contained in this web, e.g., by means of steam boxes and by thereby lowering the viscosity of the water and changing the elastic properties of the web, so that the dewatering in normal roll nips or in so-called extended nips is intensified. It has not always been possible by these means to obtain a sufficient increase in the dry solids content. Rather, even a significant proportion of the so-called free water in the web, which is not immobilized on the fiber material by hydrogen bonds, has had to be removed from the web by evaporation.
So-called closed press sections are commonly used in a paper machine, in which one press nip or, as a rule, several press nips are formed in connection with the central roll. An example of such a prior art press section is the press section marketed by Valmet under the trademark "Sym-Press II", having a smooth-faced central roll with a diameter larger than the diameters of the other press rolls and usually made of rock, as a rule of granite. Granite is quite questionable in machine construction since it is an inhomogeneous natural material of low tensile strength. If it is desired to heat a granite roll, then the deformations dependent upon temperature are non-linear and difficult to predict. As press roll material, granite has relatively good properties for detaching the web, which is at least one of the reasons for its popularity. The detaching properties however, could be better, in particular with respect to unbleached paper qualities.
In a manner known in the prior art, the web is detached as an open, unsupported draw from a face of a central roll in the press. This open draw is quite critical in view of the operation of the paper machine. In this open draw, a difference in speed is used which extends the web, resulting in certain drawbacks. Moreover, this open draw forms a web which is susceptible to breaks in a paper machine.
With increasing production rates of paper machines, the dewatering performed as nip pressing has become a bottleneck which limits the increasing of the running speeds. This comes from the fact that the press nips formed by a pair of rolls have a short area, so that with high speeds the time of residence of the web in these press nips remains short. Especially due to the flow resistance of the fiber structure of the web, the water, however, requires a certain time in order to be removed from the web into the hollow face of the roll or into the press fabric.
If attempts are made to increase the dewatering capacity in nip presses by increasing the nip pressure, with a certain linear load the limit is reached at which an increased nip pressure is no longer helpful, since the structure of the web no longer endures the compression.
In the prior art, so-called hot-pressing methods are also known, in which respect, by way of example, reference is made to U.S. Pat. No. 4,324,613, according to which the paper web is pressed in a roll nip in which one of the rolls or cylinders has been heated by means of surface heating to a temperature higher than 100.degree. C.
In this nip, the surface water in the paper web can be vaporized. The pressurized vapor blows water which has been pressed into the intermediate spaces in the fibre structure in the paper, into the press felt. The dry solids content achieved by means of this prior art, hot-pressing method is quite good, but a problem is the short nip time in a high speed machine, because the compression time in a roll nip is only about 1 . . . 3 ms, whereby the vaporization does not have enough time to begin properly, unless the roll temperature is very high (of an order of 500.degree. C.). The high temperature of the roll results in problems, in particular with respect to the strength of the press fabric and the roll.
The press treatment and the press device in accordance with the method of the present invention can be applied both to dewatering-pressing of a paper or board web and to calendering of a web, and in particular to so-called gradient calendering. With respect to calender applications of the present invention, reference is made, by way of example, to the Valmet U.S. Pat. Nos. 4,614,565; 4,631,794; and 4,653,395.
With respect to the recent inventions of Valmet connected with the press treatment of web and closely related to the present invention, reference is made, by way of example, to FI Patent Applications Nos. 871870; 870309; and 874136.
The present invention is also closely related to conductive heating of a paper web and of a press roll, reference being made in this respect to earlier inventions of Valmet connected with these applications, namely to the FI Patent Applications Nos. 870308 and 870309.
As is apparent from the above, it is known in the prior art to heat the face of a press roll or cylinder so as to provide so-called hot pressing or impulse drying, or to provide detaching of the web. An advantageous prior art mode of heating a press roll or a calender roll is external inductive heating free of contact, in connection with which, as a rule, ferromagnetic roll coatings have been used, even though ferromagnetism is not a necessity in inductive heating. Rather, electrical conductivity of the roll face in view of the heating eddy currents is what is expressly necessary.
It can be ascertained that a number of different properties are required from a roll or cylinder face to be heated, and the provision of such properties in one and the same roll face has not been solved in a satisfactory manner in the prior art. A problem has been how to provide a press-roll coating and equipment for heating the same by means of which it is possible to heat the roll face instantaneously, e.g., to about 350.degree. C. and/or to provide a sufficiently high thermal flow from the roll to the web to be heated and, at the same time, to obtain satisfactory properties of wear, thermal shock, and web detaching from the roll face. A further problem is how to permit crown-variation of a press roll in the same connection.
Induction heaters operating at a high frquency (e.g. 25 kHz) are highly expensive to construct, because of the power transistor technology required by the same.
Impulse drying requires a power transfer capacity of about 0.5 MW/m, as well as regulation of the temperature profile from a heat roll. Faces hotter than what is necessary in view of the process, ought to be avoided even in view of the fire risk alone.
A common prior art mode of solving the above problems is to pass hot gas into the interior of a press roll or calender roll. In such a case, the temperature of the inner face of the roll must be about 700.degree. C., whereby the rigidity of the roll material is no longer sufficient and crown variation cannot be effected by means of the present-day technology.
If a press roll is heated from outside by means of infrared equipment or by means of combustion gases, this would require very high temperatures which cause high heat losses and a clear risk of fire. In a manner known in the prior art, induction heating can also be applied directly to the face of an ordinary press roll made of a ferromagnetic material, but in such a case a higher frequency, i.e. technology of higher cost, is required.
The term "press roll" as used hereinabove, means both a roll of a wet press (including a roll used in impulse drying), a calender roll, and also any other, corresponding roll in a paper finishing device.